The invention relates to charged particle guns, and more particularly to field electron and ionization sources and ion guns for producing high current, medium energy ion beams, and to electrostatic lens systems for focusing the beams into small spot areas on a target.
There are numerous applications for ion guns and electron guns for use as microprobes, etc., capable of producing stable, high current, high resolution ion beams precisely focused onto very small "spot areas" of various targets. Gas phase ionization sources have been utilized but are presently incapable of providing focused/high current beams of intermediate energy which are needed for certain applications.
Interest in production of submicrometer focused ion beams for various uses has led to a demand for ion guns capable of producing high beam currents and precisely focusing the beam into very small target spot areas. High current beams, especially ion beams, have very large energy spreads. Chromatic aberration of the electrostatic lens system of ion or electron guns limits the minimum spot size obtainable with an ion or electron gun emitting a beam having a high energy spread. Very bright field emission sources, such as thermal field electron emitters, gas phase field ionization emitters, and liquid phase field ionization emitters are capable of producing submicrometer electron or ion beams with tenths to hundreds of nanoamperes of current. However, ion beams produced by liquid metal field ionization sources have been found to have much greater beam energy spreads (.DELTA.E) than gas phase field ionization sources. The high beam energy spread of beams produced by liquid metal field ionization sources limits the minimum spot size into which an ion beam can be focused by present two-element electrostatic optical lens systems for intermediate beam energies. In order to take advantage of the high level of brightness of such beam sources, it is necessary that their beams be focused by means of electrostatic lenses with low chromatic aberration in order to obtain submicrometer spot sizes. Up to now, no simple, single, electrostatic electron guns are capable of providing beam currents as high as 250 nanoamperes in submicrometer spots for intermediate beam energies in the range from 2,000 to 30,000 electron volts. In situations typical of lithography or surface analysis, where high currents are desired in focused beam spots of approximately 1,000-2,000 Angstroms, the virtual source size has a relatively small effect on the final beam diameter. This is because the large acceptance angles in the electrostatic lens system necessary for high current result in discs of confusion due to chromatic and spherical aberration of the electrostatic lens system. Such discs of confusion are large compared to the virtual source size. In many cases the contribution of the beam diameter of the virtual source can be ignored and the current and beam spot size are determined by the electrostatic lens system alone.
For many applications, a desirable characteristic of an ion or electron gun is that it have the ability to produce a focused beam voltage continuous voltage variability while maintaining a fixed image and object distance.
The advent of very large scale integrated circuits (VLSI) has brought new importance to the use of electron beam probing as a means of function testing and failure analysis. Some present day VLSI circuits "cram" half a million transistors with component detail size less than 2 microns into an area less than 100 square millimeters. Historically, when a circuit fails to meet its specficiations, its circuit elements are checked in order to localize the trouble spot and identify the cause. To do this, fine mechanical probes are placed in contact with appropriate test points where voltage waveforms are measured.
For many present day VLSI circuits, mechanical probes are far too coarse and their capacitances are so large that errors and circuit malfunctions are introduced by the mere act of measurement. Thus, an increasing demand is emerging for "nondestructive" g of VLSI circuits which imposes neither electrical or mechanical stress on the specimen. To meet this demand, two offshoots of the scanning electron microscope (SEM) known as voltage contrast and stroboscopic scanning electron microscopy (SSEM) have been developed.
All materials when bombarded with electrons (greater than 10 electron volts (eV) of energy) emit low energy secondary electrons (0-50 eV) having a characteristic distribution. If the potential of the material is made positive or negative by means of some external voltage source, this secondary electron characteristic distribution curve shifts to lower or higher energies, respectively. In a standard scanning election microscope, this shift in energy distribution with the voltage of the material shows up as a change in image brightness. For a specimen with regions of various surface potentials, the brightness variations can be used to infer voltage variations between the surface regions.
Voltage contrast scanning electron microscopy (VCSEM) allows one to use a low capacitance, high impedance probe to map out the surface voltage differences on different portions of the VLSI chip in a quantitative fashion. By proper phasing of the electron beam scan rate with the device clock signals in an energized device under test, it is possible to display the output video signal from the SEM on a cathode ray tube (CRT) in the form of two-dimensional logic maps superimposed on the IC image.
Within the same instrument concept, a further modification consisting of a high speed beam blanker allows an additional significant measuring function to be performed known as voltage contrast stroboscopic scanning electron microscopy (VCSSEM). This function allows one to follow in real time the shape and amplitude of a high frequency voltage waveform as it propagates through the various elements of a VLSI circuit. A great wealth of information that is of considerable interest when designing and trouble shooting a circuit can be obtained from VCSSEM and VCSEM.
In other applications the standing wave pattern and movement of high speed surface acoustic waves can be followed across a surface. In addition to VLSI applications, a variety of other surface studies can be envisaged with VCSEM. For example, the work function distribution of a nonuniform surface can be mapped with high spatial and voltage resolution by a raster scanning beam. Such a device can be used in a variety of applications such as elucidating grain boundaries on metallurgical specimens and measurement of work function distribution on a complex catalyst.
Unfortunately, quantitative surface potential measurement is complicated by the fact that the secondary yield is not linear with surface voltage. Even if the latter is linearized, the secondary yield is still dependent on surface topography and composition. Another factor that must be taken into account is that adjacent surface potentials have a considerable effect on the secondary yield of the measured area. Ideally, the voltage measured at a particular point by means of voltage contrast should be insensitive to surface topography surface composition, and adjacent surface potentials.
It is an object of the invention to provide a method of producing and focusing a higher current electron beam on a smaller spot area than has been previously possible, despite relatively large energy spreads that are associated with electron beams emanating from sources having high angular intensity.
It is another object of the invention to provide a method and apparatus for simplifying interpretation of voltage contrast images obtained by voltage contrast scanning electron microscopy.
It is another object of the invention to avoid complications in quantitative conductor potential measurement in integrated circuits using VCSEM techniques, which complications are caused by beam charging of passivation material over the conductor.